Automatic tuning device for radio receivers



D 16, 1941' E. N. MULLER 2,266,065l

A'UTOMATIC TUNING DEVICE FOR RDIO RECEIVERS Dec- 15, 1941. E. N. MULLER `UTQMATIC TUNING DEVICE FOR` RADIO RECEIVERS 5 Sheets-Sheet 2 Filed Oct. l0, 193B Kdnr/V5 Gew ...SNN

Dec. is, 1941.

AUTOMATIC TUNING DEVICE FOR RDIO RECEIVERS Filed Oct. 10, 1938 3 Sheets-Sheet 3 E. N. MULLER 2,266,065

Patented Dec. 16, i941 AUTOMATIC, TUNING DEvIcE Foa RADIO Amicislvlrus Egon Nicolas Muller, Esch, Alzette, Luxemburg Application october io, 1938, serial No. 234,154

In Luxemburg July 1 1936 25 claims. (o1. 25o- 40) My present invention applies to radio receivers of the type preferably wherein the tuning frequency may be` varied through a range of substantial Width, and relates more particularly to automatic control circuits for facilitating the tuning-in of the desired program.

In most receivers due to the high selectivity thereof some diiiculty Ais experienced iln lexactly tuning in the incoming signal. Of course when the receiver is not sufacientlywell in Vresonance with the carrier frequency, this will resulty in a distored reproduction. Itis well vknown that those receivers wherein the operator must rely on visual or aural indications.,V are scarcely satisfactory, since a considerable amount of care is required, which most listeners. are unwilling or unable to expend. FrequentlyA a so-called in ter-station noise suppressor means has been incorporated in modern receivers to render the tuning apparently very sharp whereby to prevent a distorted reproduction. However the operator in such eventuality might tune past the stations unless he tunes very carefully, this making the searching of the different programs very tedious. This will readily be understood when it is considered that in selective receivers a program can be adequately .listened over a very restricted range of frequencies, not much in excess of 300 cycles wide on either side of the exact position, whereas the spacing of the various stations usually is not lless than lkilocycles, even if it assumed that there are no idle channels.

According to theinvention means are provided for avoiding the above inconveniences by spreading the position of exact tuning Whereby the tuning control member, when approximately adjusted to provide the tuning-.in of a station may be moved in a moderate proportion Without causing a substantial variation of the tuning frequency of the receiver.

called A. F. C. device, which may be of conventional design, for automatically correcting departure from exact resonance of the physical tuning means i. e. the means which may be under the direct control of the operator and which are generally associated With the tuning dial; and there are also provided means for controlling the action of the A. F. C. systemn so that, when the operator tunes through the tuning band, substantially eachof the different carriers may be properly tuned Vin and released.

, Whereas proper tuning-in of the carrier is not so troublesome with a receiver equipped with A. F. C., especiallyso if vthe receiver is provided with a well-designed preselector, the operating conditions as concerns the dropping of the station are very severe since a large amount of mistuning Yof, the physical tuning means usually corresponds to a very reduced Adeparture from exact resonance ofv the actual i. e. corrected tuning frequency, and this lactual mistuning is all the smaller the more eifectiveis automatic correction; usually a large sensitivity of the automatic corrector system is desirable,

In accordance with the invention described and claimed more particularly in parent application Ser. No. 150,922, led June 29, 1937, (now Patent 2,207,467 issued July 9, 1940), the above broad objects may be attained by means of electro-mechanical apparatus. According to the invention specifically set forth in my present application, which is a continuation of parent application Ser. No. 150,922, the above broad objects maybe attained substantially in electrical fashion, mechanically moving parts if desired being entirely dispensed with.

To this end a tuning control circuit in accordance with the invention is provided with an automatic resonance Uadjusting vsystem or sofor obvious reasons. It has been suggested that reception conditions may be controlled by a circuit in substance designed along the lines of the conventional noise Suppressors. One of the major drawbacks of such an arrangement is that the initial adjustments are very diiiicult and the stability of operation of the system risks to be insucient. It is desirable that substantiallyat no time reproduction be possible with A. F. C'. in a condition of low sensitivity. A highly selective circuit is required in such case to detect the rather minute amount of mistuning which usually is left when A. F. C. is correcting a considerable amount of physical mistuning, making the arrangement expensive. O f course a minute amount of misalignment of the selective means of the suppressor circuit with respect to the frequency of the so-called discriminator of A. F. C. which serves to detect departure from exact resonance and on which the corrector stays locked, would be sufficient to make the system fully inoperative or to give rise to considerable distortion and to suppress the function of noise suppression. It is to be noted in this connection that a modest amount of misalignment between the frequency of the discriminator and the tuning frequency (e. g. the mean frequency of the intermediate frequency circuits in a superheterodyne type receiver) will not affect the operation of A. F. C. and will cause negligible distortion in the reproducer system. Another iin-,-

Fig. 4 diagrammatically shows another form of the invention, in which the automatic control circuit is made effective substantially in the same way as in Fig. 1, and wherein this circuit is blocked again by the control channel serving to make effective said tuning control circuit after the selectivity characteristics of said channel have been altered in response tothe magnitude of the automatic tuning control potential.

Fig. 5 is a graph illustrating the operation of the system in Fig. 4.

pendent on the sense of actuation of the tuning member, and in a high mean value of the track-` Fig. 6 illustrates still another form of the invention, in which the threshold of operation of ing error in the preselector circuits with respect to the intermediate frequency circuits in a superheterodyne receiver.

It is an object of the diiferentforms of circuits in accordance with my invention to avoid thevarious drawbacks above set forth and to provide among others the advantages of easy initial adjustments, entirely reliable operation under all tuning conditions in spite of minute accidental changes in the frequencies to which the different circuits are adjusted, and substantially uniform effectiveness of the control system with respect to the mean tuning frequency of the physical tuning means and tuning dial.

According to one of its aspects the invention provides means for properly dropping the station on which A. F. C. gets locked, after tuning is a desired amount away from the exact resonance point; these means are operated through received energy of carrier frequency to provide reliable operation, and their action is dependent on that of A. F. C. According to another of its aspects the invention provides distinct means for respectively switching on and off the A. F. C. channel and likewise if desired the reproducer circuits of the receiver. In accordance with the preferred form of the invention, mea-ns for switching on the A. F. C. device and if desired the audio system, are closely associated with the circuit of the A. F. C. system whereby errors due to misalignment may be avoided.

The control circuit for switching on the A. F-. C. device after a carrier Wave is tuned in, may be constructed in different Ypossible ways. I may prefer in general to utilize ay circuit capable of discriminating between a carrier Wave'and the back-ground of noise due 4vto static; thus the tuning-in ofl comparatively weak stations is permitted when the local reception conditions permit so` without the necessity of providing sensitivity control means or they like, which cannot be actuated readily enough by the user of the receiver.

Other objects of my invention will result from the following detailed description, which is illustrated in Figs. 1-7.

In the accompanying drawings:

Fig. 1 diagrammatically shows an embodiment of the invention wherein the circuit for automatically controlling the tuning frequency is made effective by means of a potential derived from a carrier wavelexceeding in a desired minimum proportion the back-ground of noise, and wherein said circuit is blocked again when the magnitude of the potential for controlling the tuning frequency exceeds a predetermined amplitude.

Fig. 2 is a graph explaining the operation of the control circuit in Fig. 1.

Fig. 3 shows a modified circuit arrangement, the operation being in the main the same asin Fig. l.

the channel for making effective the automatic tuning control circuit is normally given a rather low value, which is increased in response to the "condition of effectiveness of the automatic tuning control circuit; the operation of the system is improved by the provision of means for limiting the range of effectiveness of the tuning control circuit.

Figl. 7 graphically shows the operation of the system in Fig.' 6.

Similar circuit elements .are hereinafter given the same or analogous reference numerals.

Reference should now be had to Fig. 1, in which the' invention is arranged for use with a superheterodyne receiver. The signal energy from collector I is applied toi a tunable radio-frequency amplifier 2, the output of which is coupled to a frequency changer device comprising a modulator stage orrst detector 3 which co-operates with a tunable local oscillator'4 so as to produce oscillations of a fixed intermediate frequency. As iscommon practice in modern receivers, the different tunable circuits iny the sections 2, 3: and- 4 are uni-controlled by. a control member which may be actuated manually.

The intermediate freql'iency oscillations are applied to a tuned amplifier section 5, the output circuit of which is coupled to a further amplifier tube 9 through a transformer with a resonant primary circuit 6v and with a resonant secondary 1, both circuits being tuned to the intermediate frequency, and being moderately coupled by mutual inductance. The anode circuit of amplifier 9 includes a resonant circuit Hi tuned to the intermediate frequency. The energy across this circuit is fed through a condenser.l H18 to the demodulating detector 8, which feeds the audioampliferand reproducer. Resonant circuit I0 is also-coupled to a channel for automatically controllingy the tuning frequency within restricted limits in response to the amountV of mistunifng ofl the receiver with respect to the desired carrier wave, this device beinghereinafter referred to as- A. vF. C. system. Systems ofv this character are known in the art and it may` be stated lthat in general they comprise a so-called discriminator circuit fed with sig-nal energy to derive a control potential which is appliedy to a suitable variable impedance system hereinafter referred to as the corrector, theimpedance beingfa function ofsaid potential. In the instance shown the discriminator system by wayof example isV shown as of a well-known type including two resonant circuitsI 10i and I0-2 respectively' tuned to frequencieson one side and other' of the intermediate frequency, the amount of mistuning being for instance :t4 kilocycles, and these two circuits are coupled tov the resonant circuit Hl1 referred to above. The signal energy derivedthrough' circuit 10.2 is applied to a rectifier' H2 of the diode type, to produce a lnegative potential -acrossa load resistor 202, whereas the energy derivedthrough circuit IUI produces a positive potential across a load resistor 26| of the diode III. These two potentials are super-imposed with respect to ground to produce a resultant potential in an outgoing lead I2 provided with a lter 4|2. It is understood that when the receiver-is exactly in tune, the two part potentials cancel each other, whereas when the receiver is mistuned, at one side or other of exact resonance, one or other of the component potentials will be predominant and a positive or negative potential with respect to ground is accordingly effective in lead I2. This potential is applied to a variable impedance device associated with the local oscillator 4 and adapted for electrical control whereby to effect the required correction of the tuning.

The signal energy across the tuned circuit 6 is maintained substantially uniform despite variations in the strength of the signals at the input circuit of the receiver, by means of an automatic volume control network of conventional design, which comprises a rectifier-amplifier device I6 fed with signal-energy from the resonant circuit 6 through a condenser I9. The uni-directional potentials which are produced at the output of device I6 are applied to the amplifiers 2 and 5 through a lead I8 whereby to regulate the gain of these receiver sections.

The resonant circuit 6 is also coupled to a control channel for detecting the tuning-in and the condition of approximate resonance of a carrier wave. These means might be of the well-known type in which the discrimination between the carrier waves and static results from the proper adjustment of the operating threshold associated with the potential of a rectifier fed through a sharply tuned circuit resonating at the intermediate frequency, or the like. However the means of this general class present the important inconvenience in practice of requiring a manual adjustment of the threshold in accordance with prevailing static conditions, since an incorrect manual adjustment would upset the correct operation of the device; for this reason they are in practice suitable only for the reception of powerful stations. To avoid this inconvenience I prefer to utilize a control circuit of the kind disclosed in U. S. Patent No. 2,216,451 issued October l, 1940, which is capable of discriminating between the noise and the carrier wave of a station.

'I'he control channel just referred to includes a high-frequency amplifier tube t-he input circuit of which is coupled to circuit 6. The anode circuit of amplifier 20 includes a resonant circuit 2|, the selectivity properties of which may be moderate or low and which is tuned to the intermediate frequency. The highfrequency energy across circuit 2| is transferred to the rectifiers and 23 which are indirectly heated diodes. Rectifier 25 is arranged to produce negative potentials across its load resistor |25 whereas rectifier 23 is adapted to set up positive potentials across load resistor |23. The load resistors |23 and |25 are connected in series and the resulting super-imposed potential variations with respect to ground are transferred to the main grid 2'I of the amplifier 9 through line 26 and tuned circuit The coupling path between rectifier 25 and resonant circuit 2| comprises a condenser 24| and a network 242-243 which consists of a parallel-resonance circuit 242 shunted by a series-resonant circuit 243, both circuits being separately tuned to the intermediate frequency. .The resonance curve of such a itl network is.l double-bumped, the peaks of maximumrespon'se being for instance at 2.5 kilocycles off resonance on'both sides of the exact intermediate frequency.v 'I'hese response peaks should be Vsufficiently pronounced so that when the selectivity characteristics of the receiver portion ahead of condenser` 24| are taken into account, thepeaks-of maximum response are left sucientlypronounced, for instance at i 1.5'kilocycles-off resonance, 'with a sufficiently marked response depression at the exact intermediate frequency, the difference between the peak and dip response values being for instance 112.5. The response to the vfrequencies of the adjacentV carrier waves is preferably kept very low. Rectifier 23 is `coupled to circuit 2| through a resonant circuit 22 tuned to the intermedi-ate frequency, the selectivity thereof being preferably rather sharp, although in actual fact this requirement is not essential. Y

v The level of energy of true intermediate frequency is arranged to be' the same across both rectifiers. The circuit of rectifier 25 includes a biasing source-of potentials, the anode of diode 25 being for instance biased at 2.5 volts with respect to ground, to prevent the appearance of rectified potentials across load 'resistor |25 until the high-frequency energy impressed on the rectifier exceeds the threshold bias. The potentials across the load resistors |23 and |25 are filtered to avoid undesired inter-action between the associated circuits, the time constant of both filters being conveniently given the same value. A lter V|26 comprising a resistor and condenser may "f also be arranged in line 26. When no high-frequency energy is limpressed on the diodes 23 and 25', the potential of grid 2l is that of the biassing source of potentials associated with load resistor |25, and the cathode potential of tube 9 is moderately positive with respect to ground. The resulting negative bias of the (main) grid 21 of tube 9 with respect to its cathode is thus yconsiderable whereby the tube is normally ineffective. It will be observed that tube 9 is in the path'of signals feeding the A. 1i'. C. channel as well as the channel of the second detector, whereby normally both channels will be blocked. However if desired these channels might be controlled sepa rately.

The amplifier action of tube 9 is also controlled in response to the output of a second channel closely associated with the A. F. C. potentials of line I 2. This further channel includes a tube 2'8, to the control grid of which are applied the A. F. C. potentials of line I2. The cathode is appreciably positive with respect to ground, so that the tube ampliiies correctly both tliepositive and negative potential swings of thefA. F. C. line. Amplified potential variations appear across an anode load resistor 29, the sense of variation of these potentials with respect to ground'being reversed as comparedwith the input potentials of lthe tube. lPart of the anode potential variations is tapped off by means of a potentiometer 39-3I 'connected between the anode and a source of negative potential with respect to ground. The tap is connected to the grid of a further D. C. amplifier tube 32 and the magnitude of the potential swings is approximately the same at both 4control grids. The anode of tube 32 comprises a load resistor 33, and the amplitudes of the potential variations across the resistors 29 and 33 are approximatively the same, but the senses thereof are in opposition. The anodes of the two tubes are connected bya potentiometer including a diode' 34- in series' withV a resistor 35',

the-,cathode of the diode being connected to the of moderate time constant such` as 1A. second,

and the resultingr potentials. are applied over line- 39V to a second control grid Mlv of tube 8 to control the gain of this tube, a hexode; ora tube of similar characteristics being, for instance made use The operation of the control system in Fig. 1 is as follows:`

Assume that by means of the. tuning. control member the receiver is tuned broadly into resonance with a station of substantial amplitude with respect to the back-ground of noise. An appreciable amount of energy will bey transferred to the rectiiiers 23 and 25 to set up potentials across the associated load resistorsA i213. and 125. The excess of iniiuence of rectifier 23 will reach a sufficient value to bring the potential of line 26: more nearly toward ground potential. Tube 9 will thereby be permitted to4 pass. a small amount of energy to the A. F. C. discriminator channel. The small resulting potential in line l2 will bring the receiver near to the exact. resonance point, and the potential of line 26 will be` closer to ground potential, whereby tube 9 may amplify more effectively. A trigger action thus rapidly builds up, and line 26l will. speedily be substantially exactly at ground potential.r It will be observed that since at exact resonance equal amounts of energy are eective across both rectifiers 23 and 25, their influences will mutually cancel exactly;v the potential of line 26 as well as the operation. of the system do not therefore depend on the effectiveness of the A. V. C. de-

vice, and the potential variation of line 26 a certain distance ofi resonance is solely defined by the characteristics of the, networks associated with the rectiiiers 23 and 25.`

It will be observed that when the receiver is toc considerably off resonance with respect to a carrier wave, no negative potential will be set up across biased rectifier. 25, whereas positive potentials will still be produced across rectiiier 23. By virtue of the provision of resonant circuit 22 these cannot however reach a sufli.- cient value to bring about'the e'ectiveness of tnbe 9.

The receiver is prevented from responding to intense static only, in the following way. Since the noise energy is rather uniformly effective over a wide range of frequencies and since the feed path of rectifier 23` is predominantly effective at or about the exact intermediate frequency, it will be clear that the output of rectifier 23 will in the mainV be defined by the intensity of "static of the intermediate frequency, the effect thereof being substantially that of a carrier wave having exactly the intermediate frequency. On the other hand since the feed path of rectifier 25 is predominantly effective at frequencies moderately spaced from the intermediate frequency and more particularly at the peak response frequencies, the potential due to static for a certain amplitude of static at the input of the receiver will predominantly be defined by the sensitivity of response at about the peak frequencies, the effect being rather analogous to that of a carrier wave moderately off' resonance as considered above. Since the sensitivity or response at these peak frequencies is` comparatively higher than the sensitivity at the intermediate frequency and since the latter response is approximatively the same as the predominant response at the intermediate frequency across rectifier 23, there will be a predominant influence of the negative output of rectifier 25. The grid bias of tube 9 cannot therefore reacha suiciently lowered negf ative bias, so that static will never cause the erroneous operation of the device. Furthermore the excess output of 25 is practically constant whichever may be the intensity of static and depends only on the ratio of peak to crevasse response of the resulting resonance curve characterstic effective across rectifier 25.

Brief interfering impulses due to atmospherics or to sparking engines or the like will simultaneously set up potentials across both rectiiiers,

l the potentialdue to rectifier 25 being somewhat higher than in the above case of uniform static, and they cannot therefore cause an erroneous.

operation of the control channel.

When astation is exactly in tune, line 26 being at ground potential as indicated in the foregoing,

' the anodes of the tubes 28 and 32 and the tap of potentiometer 2id-35 will assume a predetermined reference potential, which isA such that the striking voltage of the neon lamp 36 is apprev ciably exceeded. A potential is accordingly set up-across resistor 33'; this potential may be rather the two associated anodes which has the relatively lower potential. For instance when the potential of thev anode of tube 28 decreases with respect to its reference Value, diode 34 is conducting and its resistance is low as compared with 45 the resistance of resistor 35. When the anode 55 negatively so as to inhibit the amplification of tube 9. This control action may take place when the A. F. C. potential tends to correct an amount of mistuning of some *2.5 or 3 kilocycles. At the instant at which tube 9 is being 6.0 disabled,l the amount of physical mistuning just indicated may be reduced to a low value of actual mistun-ing of some 50 cycles.

After tube 9 has been blocked again, the corrector action disappears rapidly since the time constant of A. F. C. line l2 is very small, e. g. l/oth second. The time constant of filter 31 in line 35 for blocking again tube 9 is given a larger value so that the neon lamp may be lightened rapidly as soon as the potential of A. F. C. line I2' has reached a value suiciently close to ground potential, without the blocking action of line 39 being thereby aiiected. However after about 176th second the Vbias of grid 40 also is again close to its initial value. However tube 9 cannot now amplify by virtue? of the excess bias of grid 2T frequency,

of the next station.

-. e The above operation of the receiver will be more fully understood with reference to the double graph in Fig. 2, in which the left-hand portion shows the actual tuning frequencey of the receiver on the ordinates whereas the potential due to a carrier wave of line 26 is indicated on the abscissa-e. The right-hand portion shows the actual tuning frequency (vertically) as a function of the physical tuning setting as shown off by the tuning dial, the tuning frequencyvarying in the sense of the arrow. The A. F. C. channel is rendered effecf tive as soon as the potential of line 26 exceeds the threshold value s, and the actual amount of mistuning, which was considerabl-e, falls to a very low value. At the instant at which the A. F. C. device is blocked again vafter mistuning L.. has reached an excessive value, the amount` of physical mistuning-is somewhat largerv than the initial value, so that the threshold s may not be exceeded again, whereby the device is now prevented from being controlled-again in response to the carrier waveoriginally tuned in.

-The arrangement shown in Fig. 3 serves to .illustrate some modifications in the disposition of the channels for, producing the potentials responsive to the selection of the carrier wave`t and responsive to the magnitude of the A. F. C.

potential respectively, and, of the means for applying these potentials to control the action of the A. F. C. device.

The control channel for producing a potential 7 in response to the broad selection of the desired carrier wave includes the amplifier tube 20, which is fed from the intermediate frequency circuit 6 whereat the signal level is substantially uniform as in the foregoing arrangement due to the automatic gain control device I6, and the ymediate frequency whereas the circuits '44 and4 46 are tuned separately to frequencies on one side Vand other respectively of the intermediate frequency, mistuning being about 2.5 or 3 kilocycles. Rectifier 23 which produces positive potentials is associated with a tuned circuit 42- which may be flatly resonating at the intermediate frequency," and which is coupledv by mutual inductance to resonant circuit 4|; Vthe relative response across this circuitfdoes not depend appreciably on the response across the further circuits 44, 46. The biased rectifier which produces potentials of negative polarity is coupled capacitatively to the junction of the resonant circuit 4| and of the series-connected circuit 44 and 46. The relative response across the series-connected circuits 44 and :46 presents a substantial value at frequenciesmoderately spaced from the intermediate frequency, and a very l'ow response at the exact intermediate and it will be observedv that the response curve characteristic of network f4l-44-46, from the point of View of rectifier 25, is substantially similar to that of network 242-243 in Fig. 1. The super-imposed potentials `ofthe re'ctiiiers' 23 and A25 serve to control the gain. of the tube 9| of the A. F. C. channel, as detailed hereinafter. v

The control channel 4for producing a potential responsive to the magnitude of the A. F. C. potentialv includes a D. C. amplifier tube 3|,r which may be considered as performing the functions of the tubes 23 and 32Y in Fig. 1; tube 8| is provided with a cathode resistor 292 and with an anode load resistor 29|, which are connected between the cathode and ground and between the anode and the high tension supply respectively. The A. F. C. discriminator schematically indicated at I I-I I2 is of conventional design and may for instance be constructed as illustrated in Fig. 1. The output line I2 of the A. F. C. device is connected to the grid of tube 8| and is effective between the gridV and ground. The potential variations across the cathode load resistor 292 are approximately those of the AfF. C. potential, the gain of the tube effective across this load resistor being slightly less than unity and not dependent in practice onthe resistance valu-es ofthe cathode and anode-load resistors. 'Ihe resistance'of `anode load-resistorf29| is considerably higher than the resistance ofcathode load resistor 292,'and in a practical embodiment may be approximately ten times higher, and vthe magnitude of the A. F. C. potential across load resistor 29| may laccordingly be about 10 times higher than across load resistor 292; furthermore the polarity of the potential changes is reversed, with respect to the variations of the A. F. C. potential across the input oftube 8| and of the potential across the resistor 292. uThe anode of tube 8| is connected to ground through Va potentiometer formed by the series-connected resistors 41 and 48, the resistance value of resistor 4'! y connected to the anodev being about 9 tim-es higher, in the `numerical'in'stance just considered, than the resistance Aof resistor 48 -connected to ground; thus the magnitude of the potential Avariations effective between ground and the junction of resistors 41, V48 maybe about lyoth the magnitude available across the anode load resistor 29|, and maybe about'v the saine as thev magnitude of thepotential changes resistor 292 the junction of said diode and resistor may have its potential defined by the lowest potential relative to ground which may be assumed at any particular instant by th'e cathode and the junction 4'I'-48 respectively. In the embodiment shown, the diode 50 is enclosed in the same'bulb vas the amplifier tube 8|, and utilizes part of the common cathode sleeve construction, thermionic tubes of such construction being well-'known in the art. Y

To improve the operation of the control channel' which causesthe A.` F. C. device to resume the ineiiective condition, means Aare provided whereby after 'the potential of` line 5| -justre- 1 ferred to and responsive to the magnitudeof the A. F. C. potentials has reached a certain critical value, a minute further change of that potential caused byan increased magnitude'of the A. F. C. Vpotential may give rise'to a considerablevariation of the control potential causing the blocking of the A. F. C. device. In the embodiment'sh'own these means rely Yon the considerable Variation of the internal Vresistance of a diode indicated at 52 (or of some other rectifier) in response to the impression thereacross of a certain critical potential. The associated circuit isrof the type disclosed in the speciiication of Luxemburg Patent No. 22,997 issued March 3, 1937, (see also U. S. application No. 181,317 filed December 23, 1937). Diode 52 is connected in the grid circuit of an auxiliary amplifier tube 54 which may amplify at high-frequency; the input of tube 54 is coupled toan auxiliary generator 55 of A. C. .potentials of a desired frequency which is not critical; such a generator in many cases may'serve for various other purposes in the receiver. Diode 52 is connected in series with a resistor 53 of high resistance value; the free end of the resistor is connected to line and the anode of the rdiode is connected to the other terminal of the resistor, while the cathode of the/diode is connected 'to a point of iixed potential 56 which is lower, by a predetermined amount, than the reference potential of line 5|, i. e. the potential assumed in the absence of any A. FIC. potential. The junction of diode 52 and resistor 53 is connected to the grid of tube 54; the cathode potential of tube 54 is a few volts hgher'than the potential of point56'which latter may conveniently be tapped off from the cathode circuit of tube 54. The output of generator 55 is connected to the-grid of'tu'be '54 through a condenser |54 and a resistor 51 of a comparatively high resistance; this resistor from the point'ofview of the yhigh-frequency voltages l'erfectua'lly 'forms a potential divider including diode 52 `and resistor 53 as the second impedance arm, the two latter impedances fb'ein'g effectu'ally kconnected in parallel from the high 4frequency Apoint of view. Since as stated above the'j-potentialfof vthe cathode of diode 52 4is'lower than'the reference potential of'line 5|, 'the vdiode is normally conducting and its'internal resistance is low with respect to the resistance of 51,`so that the TA. C. voltage effectively impressed upon the Vgrid of tube 54"maybe very low. "This condition is le'ft unchanged when 'the potential Aof line 5I decreases moderately with resp'ect to the reference potential. y

The 4anode circuit of'amplin'er 54 `isprovided with a Yload resistor', :fand the voltage across rth-is resistor r is 'applied to rectifier 59 of the diode type through leadSI-'L The're'ctifled'potent'ial of negative polarity which is produced across load resistor `60 fof 'this rectier is derived through fa-resistan'ce-capacity lter 6| and serves tocon'trol 'the action ofthe A. F. C. device, 'and -more 'particularly the :gain of ampliner tube 9| in the high-frequency path thereof. Normally `there isno appreciable 'potential across load re- 'Ihe operation as a wh-ole of the circuit in FigfV 3 will kbe understood more easily with reference to Flg. '.2 and 'is as follows: By a proper adjust- `ment of the cathode potential of tube 9| it iis `arranged that lthis tube 'isbiased vto cutoi in the 'absence o'f a carrier wave, the 'grid potential 0 being defined by the `delay bias 'of rectiiier 25. 'This Acondition is left unchanged duringthe reception of static 'whichever may be the intensity "of reception, since the peak response across rectifler 25 is higher thanthe peak response'across rectiiier 23, 'wherebythe negative rectied potential across '25 is'predominant. If the receiver is tuned broadly to the frequency of a carrier wave, the amount of mistuning being lower than some 2.5 kilocycles, the input of rectifier 23 may become nearly as high as the input of rectier 25, and the super-imposed potentials may cause the grid of'tube 9| to reach a potential slightly less than ground kpotential and may reduce accordingly the negative biasthereof. Tube 9| may now amplify in a Yreduced proportion and a certain A. F. C. potential may vbecome effective, which improves the tuning of the receiver. Due to the consequent improvedresponse across rectifier 23, a further decrease of negative bias of tube 9| vmay'take place, and normal operation of the'tube may be ensured speedily. The A. F. C. device and the reproducer channel 8 under these conditions may operatefcorrectly Since the misalignment fof -the Aphysical 'tuning means is supposed to be rather considerable, acomparatively high contrel potential will be applied to the control grid oftu'be 9|, andthe potential of line 5| will be decreasedina substantial proportion. Point 56 is arranged reliably to have a lower potential than said line '5| so that diode 52 may be left conducting, vand 'there will consequently be no appreciable potential across resistor 6|) susceptible of changing-the grid bias of tube 9|. Now suppose that the amount of mistuning is increasing appreciably, the operator for instance ytuning past the station, as indicated by the arrow .in the right-hand `portion of the graph in Fig. 2. The magnitude of the A. F, C. potential will be increased, and'according as this potential is negative or positive with respect to ground, the potential of 'the cathode of 'tube 8| or of the .junc- .tion 41-'248 will decrease appreciably with respect :totthe referencevalue (i. e. its potential in the absence of "a signal) .Y In the lirst case the "diode Vwill be conducting, whereas in the second case itwill 'be non-conducting, and in either case the ydecrease of -potential will be transferred to line 51;' as Athe magnitude of the A. F. C. potential freaches va suiiicient value, the potential of line 5| -may become .more considerably negative than point 56, and diode 52 may become non-conducting or at least yits internal resistance may be increased very considerably, so that the effective lresistance in fshunt across the grid-cathode path iof amplifier tube 54 may be increased considerably and an appreciable A. C. voltage from source 55 may become eiective across the grid and the cathode. This A. C. voltage after amplification in the tube is appliedto rectiiier 59, and the thus produced negative potential substantially increases the negative bias of tube 9|.

The effectiveness of the A. F. C. device may thereby-be-considerably lowered, and the resulting higher amount of mistuning of the receiver mayicause asubstantial drop of potential of line 26, hence a further decreased gain of tube 9|, which in 'fturnicauses a further increased amount 'of mistuning. The A. F. C. device may thus be rendered .ineffective 'speedily and the reproducer `channel f8 may simultaneously be restored 'to `the inelective condition.

:As .the A. F. C. potential is disappearing, the .potential of line 5I simultaneously is reaching .again Vits reference value, vand the high-fre- `fluency energy due' to source 55 applied to rectifier 59fmay speedily disappear, while the potential :of line 26 still is continuing to decrease as set yorthfabove. However an appreciable negative potential is maintained for a brief period of time such as 1/5th second across the condenser of lter 6| associated with rectifier 59, and serves to ensure a sucient reliable operation by disabling amplifier 9| until the potential of line 26 reaches its reference value. Y

Reference should now be had to the alternative form of the invention illustrated in Fig. 4. The control channel for producing a potential in response to the tuning-in of a carrier wave includes an amplifier tube 20|, to the main control grid 60 of which is applied signal energy from the intermediate frequency amplifier 5, through the resonant circuit 6 whereat the signal level is maintained substantially uniform by means of the automatic gain control network I5. The rectifier 23 may fbe considered as coupledto the anode circuit of amplifier tube 20| through a transformer with a resonant primary 4| and a resonant secondary 42, both circuits being normally tuned exactly to the intermediate frequency. The negatively biased rectiiier 25 is coupled capacitively to the anode circuit of tube 20| through the intermediary of a network including the series-connected resonant circuits 4|, 44, 46, the two latter circuits being tuned to frequencies on one side and other respectively of the intermediate frequency to provide a double-humped resonance curve characteristic with a suiliciently pronounceddip or crevasse at the intermediate frequency. It will be observed that the disposition just considered is quite similar to those shown in Figs. 1 and 3. The rectified potentials across load resistors |23 vand |25 of rectiers 23 and 25 are of positive and negative polarity respectively and are super-imposed in the lead 26; these superimposed potentials are utilized to render effective amplifier tube 92 in the feed path of the A. F. C. device in response to the broad selection of a carrier wave, through the intermediary of a device represented schematically and marked trigger device, having its input connected to line 26, and its output connected to the lower terminal of circuit 1 in the grid circuit of tube 92 through a resistance-capacity filter |31 having a rather small time constant and through lead 226.

This device serves to provide a comparatively sharp variation in the amplitude of the control potential derived through lead 26, when said potential assumes a certain critical value, whereby the grid bias of tube 92 may be `altered comparatively abruptly and the operation of the system may be improved. This trigger device may be of any well-known construction and may perform substantially the function of neon lamp 36 and resistor 38 in Fig. lor of the thermicnic circuit in Fig. 3 including auxiliary A. C. source 55, tube 54, impedances 52, 53, 51 and rectier 59.

The A. F. C. discriminator I2 is coupled to the anode circuit l of amplifier tube 92 and may be of any well-known construction, for inance with the amplitude of the A. F. C. potential,

at a desired high rate and in the same sense as, or in the sense opposite to, that of the oscillator circuit. Mistuning of circuit 42 is toward one side or other of the intermediate frequency according as the A. F. C. potential, responsive to the small mistuning of the receiver in the corrected condition thereof, varies in the sense of positive or negative changes out of the reference potential condition; it will be understood that in either case the level of signal energy transferred through circuit 42 is decreased as the magnitude of the A. F. C. potential and the resulting amount of mistuning of circuit 42 is increased.

VWhereas a great variety of arrangements may be used to cause the said mistuning of circuit 42, including purely electrical circuit arrangements for instance relying on the so-called Miller effect, it is believed that these circuits are sufficiently well known in the art, and for the sake of simplicity electromechanical mechanism is used including a motor of the galvanometer type indicated at 14. The galvanometer coil is included in the anode circuit of a D. C. amplifier tube I2 I,

the grid of which is connected to the output line |2of the A. F. C. discriminator III-I |2. The spindle |4I of the motor 14 is grounded and carries the moving vane of a condenserv 215 connected in shunt with the principal condenser of resonant circuit 42 through lead |15. The movement of the motor spindle may simultaneously control the oscillator frequency of the frequency changer device by means of a similar condenser 315.

The operation of the circuits in Fig. 4 will no-w be considered, reference being also had to the double graph in Fig. 5, wherein the right-hand portion shows the actual tuning frequency of the receiver as a function of the tuning frequency dened by the physical tuning means of the receiver, and the left-hand portion shows the potential of the channel responsive to the tuningin of the carrier wave with respect to the associated operating threshold s indicated by the vertical dash-line as a function of the actual tuning frequency, the abscissae andA ordinates being similar to those in Fig. 2.

Tube 92 is left excess-biased while no signal is being received, the A. F. C. device as well as the receiver portion 8 fed from the outputof this tube being thereby inoperative, and the tuning frequency of the receiver being solely defined by the normal or principal tuning means. Just as in Fig. 3, conditions are not changed in this respect when static is received.

However when a signal is broadly tuned in, mistuning being less than some 2.5 kilocycles, high-frequency energy of substantial amplitude which may be slightly less than the level of energy at rectier 25 may be applied to rectier 23, and the resultant rise of the potential of line 26 is transferred through the ftrigger device and through filter |31 to the grid of tube 92, whereby to remove the cutoff bias from this tube so that thereproducer channel 8 maybe'- come effective as well as the VA. F. C. device. By the way it will b'e observed that While the excessbias of tube 92 is being lowered or removed as just indicated, a small A. F. C. potential may take place which on account of the thereby improved resonance conditions may increase somewhat the level of energy at rectifier 23 and decrease the level of energy at rectifier 25, thus bringing about a `further increased potential of line 26 which in turn is fed back to the grid of tube 92 and which is cumulative with the action of thetrigger device.y Normal operating conditions of tube S2 may thus be attained 'still more speedily.

kIt will be observed that at the instant of tuning-in of the carrier wave, the effective response curve of the control channel including tube 2M and represented by the potential variation of line 26 with variations of the tuning frequency around the resonance frequency, may be indicated by the plain curve at the left of Fig. 5. During the small fraction of a second in which lthe actual tuning frequency of the receiver is brought into resonance 'with the received signal .through the A. F. C. device, the effective response curve of this channel is however modified due to the mistuning ofthe resonant circuit t2 through the A. F. C. potential; it is arranged however that the corresponding decreased response of the control channel with respect to the associated operative threshold may reliably continue to exceed the said threshold, as long as the amount of mistuning, supposed above to be about 2.5 kilooycles, is not further increased.

Now suppose that the misalignment 'of the physical tuning means of the receiver is further increased, up to say, 3 kilocycles, as when the operator tunes through the `exact resonance point and sufliciently past that point. On account of the resultant considerable (and progressive) mistuning of circuit 4t2 by the A. F. C.

device and condenser 215, the effective response curve of the control channelincluding the said circuit may be that indicated by the dotted line at the left of Fig. 5. It will `be appreciated that progressively the response may become insumcient to further exceed Vthe operating threshold s, which may be the operating threshold of the trigger device in Fig. 4.' The level of signal energy across Arectifier 213 may thus become in- -suiiicient as compared with the energy level across rcctiier 25, and the thus decreased potential of `line 26 causes a still more substantially decreased potential of line 225 which may be effective with the desired abruptness and may speedily ensure that the `initial excess-bias of tube 92 is restored. The A. F. C. potential may therefore disappear speedily, whereby the initial conditions-as concerns the selectivity of the amplier channel responsiveto the tuning-in of the signal maybe restored, and the said channel may quickly have insuicient response to the signal which was vtuned in. It may be observed that until this condition is effective i. e. until the full amount of mistuning dueto the physical tuning means becomes eiective, the amount of m'istuning of the receiver with respect to the sig; nal that was tuned in will increase whereas the amount of Ymistuning of circuit v1.12 will simultaneously vanish; accordingly the delay iilter |37 serves to maintain in the meanwhile tube 92 blocked. The potential across this filter may disappear after say 1/5'th second, whereby the receiver may 'resume its initial condition.

Referring now to the form of invention shown in Fig. 6, the A. F. C. device includes an amplifier 93 fed with .intermediate frequency energy through the resonant transformer 6 1. rihe anode circuit comprises a resonant circuit I tuned to the intermediate frequency, coupled to the rectiers ||I and ||2 of the A. F. C. discriminator through the resonant mistuned circuits Il and |02 respectively. These rectiers are provided with load resistors and 282,

`eieotually connected in series and serving to derive potentials of relatively opposed polarities; the super-imposed potentials for A. F. C. control 4are derived through lead I2 and the disposition is substantially as shown in Fig. 1.

It is arranged that the A. F. C. device may have a limited range of eiectiveness, with a view to -leaving a suiciently appreciable amountrof mistuning when the physicalA tuning means of the receiver Iare mistuned by `several kilocycles, so that the operation of the control device may be facilitated. The sensitivity as a whole of the A. F. C. device might lbe low to this end, at all times, but this is undesirable, and .it may therefore be arranged that a reduction of 'the sensitivity takes place more particularly While the physical tuning means are actuated, as by causing such reductionin response tothe operation of the tuning knob or other operating means of the principal tuning means, by mechanism well known per se. However I have preferred to ensure a sensitive operation of the A. F. C. .device as long as the vamount of mistuning ofthe physical tuning means is nctexcessive.

The A. F. C. control line to this end is associated with two limiting devices for limiting the amplitude of the A. F. C. potential derived through line |2 in the sense of positive and negative potential variations, including respectively a diode -8| associated with a resistor 80, `and a diode 83 associated with a resistor 84. The series-connection of the diode 8| and resistor 80 is connected between the output line 'I2 of the A. F. C. discriminator and a point `82 of xed potential, -moderately positive with respect to the reference potential of line `I'2, i. e. the potential of that line in the absence of a'carrier wave, `and which as shown is at ground potential. As shown the potential of point 82 is tapped off from a potential divider connected across the terminals of the high-tension supply of the receiver, a desired intermediate -point of which is grounded. The-cathode -of diode 8| is connected to line |2, and the free .end of the resistor is connected to point 82, whereby the diode vmay normally-be conductingandrmayfhave a very low internal resistance. The junction of diode l8| and `01E resistor PBl) -is connected lto a point 85 kof ixed potential, .moderately negative -With respect to :the reierencepotential .of the A. F. C. line, through the series-connection of diodef83 and resistor 84. The anode of diode 83 is connectedto the junction Yof Y--8|, Vthe free end of 4resistor 84 being connectedto .-apoint-85 .which may be tapped 01T from the potential ldivider just referred to. Diode 83 may accordingly 'be normally conducting and may have 'a very ylov/'.inIernal resistance. The A. F.-C.1p0tentialof the junction of diode 83 and resistor .-84 is applied .through line 2|2 to control the=oscillator frequency of the receiver in Well-known fashion.

The Vcontrol channel .for producing a potential serving to make eiective the .-A. F. C. .device .in response to the broad .selection of a-carrier wave includes an ampliier tube .202, .the normal control grid of which is fed with intermediate frequency 4energy from .ther resonant circuit 6 whereat the .signal le-vel is `maintained -substantially constant by .means of the .automatic gain control device I6. Theianode circuit of tube .202 comprises a flatly resonant |circuit 2| separately coupled tothe rectilers -23| and i25| through the intermediate-frequency circuit .22 and through the network 242-243 respectively; this -network has a suiciently pronounced double-humped resonance curve characteristic, and the disposition is similar to that of corresponding elements in Fig. 1. j

The biased rectifier 25| produces potentials of positive polarity vacross its load resistor |25|, whereas rectifier 23| produces potentials of-negative polarity 'across the fassociatedgload .resistor [23 which is eiectually connected in series with' load resistor |25I, the super-imposed potentials being derived through line 26 l; Whereas the potenltial of this line is normally slightly or moderately positive with respect to ground due to the biasing source of potentials of rectifier 25|', the same is caused substantially to assume ground potential when the desired signal is eX- actly in resonance, the level of intermediate frequency being in substance `the same across the rectifiers 23| and 25|.

Line 26| is connected to the normal control grid of the amplifier tube 202 through thenormal grid leak thereof, tube 202 being accessorily utilized to perform the function of a D. C. amplier. The cathode of tube 202 is at a desired positive potential, so that the control grid of the tube may normally be biased slightly or moderately negatively relative to the cathode, whereby the tube may eiiciently amplify the signal energy impressed upon its input. The lower terminal of resonant circuit 2| in the anode circuit of tube 202 is connected to the positive terminal of the high-tension supply though a D. C. load resistor 81, which is by-passed from the high-frequency point of View. Between the junction of resistor 81 and the lower terminal of circuit 2| on one hand, and the negative terminal of the high-tension supply on the other hand, is connected a potential divider formed by a neon lamp 80 connected in series with a resistor 88, the free terminal of the resistor being connected to the negative potential. Since there is normally a comparatively substantial potential drop across the D. C. load resistor 81, the junction of the neon lamp and of resistor 81 is normally at a comparatively moderate potential with respect to the negative terminal ofthe high-tension supply, and the neon lamp may normally be extinguished, so that no current may ow through the associated resistor 88. The junction of the neon lamp 83 and resistor 88 may accordingly assume normally the potential of the negative terminal of the high-tension supply, and is connected to the control grid ofv amplifier tube 93 in the feed path of the A. F. C. device through a resistance-capacity filter 92 and through the normal input circuit 1 o f tube 93. The cathode of tube 93 may be at the desired potential, adjusted in such a way that tube 93 is normally biased approximately to cutoff.

The lower terminal of the tuned circuit I in the anode circuit of amplifier tube 03 is con nected to the highftension supply through a D. C. load resistor 89. The junction of this resistor and of the 'lower terminal of circuit 0 on the other hand is connected to the negative terminal of the high-tension supplyV through a potential divider' comprising the series-connected resistors 90, 9| The junction of these resistors is con? nected through a filter 92a to Van auxiliary con'- trol grid of tube 282, of Which'a decreased potential brings about an increased anode current, the said auxiliary control grid being for instance the screen grid of a tetrodeY or a similar tube being used at 202, the attainment of such a tube characteristic with conventional tube constructions under suitable operating conditions and more particularly when using a not too low grid potenthere will lbene potential drop Vacross vload 14esistor' 89.

The 'operation .of the system shown in Fig. 6" will now be considered, reference being also had to the double graph in Fig. '7, the right-hand portion of whichshows the actualtuning freey quency as a function of the tuning frequency dened by the physical tuning means (i. e. in the absence of the tuning corrector means), and the left-hand portionof which shows the amplitudeo'f the control potential of line 23| or of the. output of the D. C. amplifier 202 as a func'-v tion of .the actual tuning frequency.

Tube 93 isv excess-biased in the absence'of a carrier-wave, and the A. F. C. device andthe reproducer are inoperative. When high-frequency energy due to static is impressed upon theinput of the receiver, energy will'predominantly be impressed on rectifierl'l,` as compared with the energy impressed on rectifier 23|, and the negative grid biask of tube 202 will be somewhat de creased, whereby the resultant, even more intense anode currentof-tubel 202 causes a substantial potential drop acrossresistor 81 and leaves the neon lamp86 eXtinguisheLthe amplier 93 beingaccordingly left excess-biased.

However when the receiver is broadly tunedto' the frequency of a carrierv wave','the amount ofmistuning being less than say, 2.5 kilocycles, the highefrequency' energy.' applied to rec'tiiier 23 will be but slightly 'lower than theV energy applied to the biased rectiiier 25|', andthe consequent decreased potentialV of line 26| will increase thei negative bias of the normal control grid of tube 202 and will cause a decreased anode current and an increased .potential of the tap 88-81; this may be represented by the full line at the lower lefthand portion of the graph in Fig. 7. The threshf; old-value of the system, represented by the ver-.- tical dash-,line marked's, at the left o'f Fig. '7, and representing in the construction of Fig.'6` the striking' voltage of neon lamp 86, will be eX- ceeded, wherebythe neon'lamp will be lighteneclv and a current of substantially predeterminedintensity 'may traverse resistor 88. The junction of Sttogether with the associated grid po-v tential of tube 03 may thus risesuiliciently to cause normal amplication of tube 93, hence normal operation of the A. F. C. system and normal' reproduction of channel 8. `It will be noted that both Idiodes 8| and 83 areconducting under these conditions; the potentials 32, 85, are to'this end adjusted in such a way that potential 82 maylre-` liably be more considerably positive or potential 85 more considerably negative than the potential of lead 2 as defined bythe vmagnitude ofthe A. F. C. potential when' the signal is broadly se-I lected; the outgoing control line as applied to the A. F. C. impedance (not shown) is accordingly virtually connected directly to line |2.

\ The receiver is now speedily brought insubstantially exact resonance with thev carrier wave, due tothe action rof the correcting A. F.C`.V impedance, and the response across rectifier 23| is slightlyv increased whereas the ,response across rectierzl is slightlydecreased; this causes a further decreased potential ofline 20| Yand aV further increased potential of junctionj 86-81;` the bias of tube S3 may however 4be left substantially unchanged. On the other hand as spugna as the excess-bias of tube 33 is rernovedgahsuie.;

'stantial anode current i'lows throughv loajlid 5% sistor 89 and lowers the pot ntiafmgfthen u,

tions `:isct and sra-91,5 arte?, fgytrifgjfqenm due to citer 92a, the auziii pesanti-grief? tube zu: has its potentiaimwereaandthe an increased potential drop across resistor 81, hence a decreased potential effectively applied to the neon lamp, which may nearly compensate the decreased potential drop which tends to take place on account of the improved resonance condition considered above. As will be seenV more particularly with reference to the left-hand portion of Fig. 7, despite the fact that the receiver is substantially exactly in tune with the carrier wave, the threshold value s indicated by the vertical dash-line is but slightly exceeded, and a comparatively low amount of mistuning with respect to exact resonance is suilicient to cause the neon lamp to be extinguished. However on account of the action of the A. F. C. device illustrated more particularly inV the right-hand portion of the graph, this small mistuning corresponds to a comparatively large amount of mistuning of the physical tuning means. The` actual amount of mistuning of the receiver may be as low as 50 cycles as long as the amount of'misalignment of the physical tuning means is lower than some 2.5 kilocycles; this is shown by the slightly incurved and nearly horizontal response curve portion in the right-hand portion of the graph. However after the amount of misalignment of the physical tuning means is further increased, Ythe corresponding increased magnitude of the A. F. C. potential causes the potential of line I2 to exceed, in the sense of positive or negative potential changes as the case will be, the potential either ofpoint 82 or of point 85. One or other of the diodes 8| and 83 may accordingly be effectually connected directly to point 82 or point 85, as the case will be. Any further mistuning of the physical tuning means of the receiver cannot therefore be compensated in appreciable proportion by the A, F. C. device; this is represented by the considerably incurved portion of the response curve in the graph, this portion tending to become parallel to the response curve portions representing uncorrected tuning.

As the resulting actual mistunin'g of the receiver reachesv a value of about 500 cycles, the correspondingly increased potential of line 26| and decreased potential of tap 86 causes the neon lamp to become extinguished and thus causes the suppression of the potential drop across resistor 88; this latter potential variation is rather quickly transmitted through filter 92 to the grid of tube 93 to cause excess-bias. The reproducer channel will now be muted again, and the A. F. C. device will rather abruptly become ineffective, making the full mistuning of the physical tuning means effective, as shown by the vertical response l curve portion in Fig. '7.l The potential of control line 26| may new speedily resume its reference potential to define a comparatively low negative bias of the main control grid of tube 20L-thus causing a substantial anode current; On the other hand the decreased anode current of tube 93 taking place as this tube is again excessbiased, speedily causes a rise of the potential of tap Sli- 9| however the corresponding increased potential of the auxiliary control grid of tube 202 tending to decrease the anode current is delayed by lter 92a during 1/5th of a second, say, so that a sufficiently low negative bias of the main control grid may be ensured as explained above, and the anode current may be left suflicient to maintain the neon lamp extinguished. The system is now ready for a fresh sequence of operations. i

Having thus described certain typical embodiments of my invention, I declare that what I claim is: l

1. In a radio receiving system, the combination of tuning means, automatic resonance adjusting means for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means; means for maintaining said automatic resonance adjusting means ineffective during the absence of a carrier; carrieroperated means for causing the eiectiveness of said automatic adjusting means in response to the broad selection of a carrier by said tuning means; a second carrier-operated means for controlling the action of said automatic resonance adjusting means, applied to release said carrier and operated in response to the reception and departure from exact resonance of said carrier; means for leaving said second carrier-operated means ineffective during the absence of a carrier; and means for rendering saidsecond carrieroperated means effective substantially concurrently with said automatic adjusting means. 2. In a radio receiving system, the combination of tuning means, automatic resonance adjusting means for bringing thefreceiver into substantially exactV resonance with a Acarrier broadly selected by said tuning'means; a reproducer channel for utilizing the modulation; conveyed by said carrier; means for maintaining said automatic ad-` justing means and said reproducer channel in.- eiective during the absence of a carrier; carrieroperated means for causing the eliectiveness of said automatic adjusting means in response to the broad selection by said tuning means of a carrier; further carrier-operated means for respectively controlling the action of said automatic resonance adjusting means to release said carrier, and for controllingthe condition of reproduction of the modulation in said reproducer channel, said further carrier-operated means operating in response to the reception and departure from exact resonance of said carrier; means for leaving said further carrier-operated means ineffective during the absence of a carrier; and means for rendering said further carrieroperated -means effective substantially concurrently with said automatic adjusting means.

3. In a radio receivingA system, the combination of tuning means; automatic resonance adjusting means for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means; a reproducer channel for utilizing the modulation of said carrier; a carrier amplier stage fed from a desired point in the receiver, having its output connected to the input of both said automatic adjusting means and said reproducer channel;v means for minimizing the gain in said stage during the absence of a carrier; a rst carrier-operated means, for making ineffective the action of the gain-minimizing means in response to the broad selection by said tuning means of a carrier; a second carrier-operated means for -controlling the gain of said stage, applied to minimize the gain regardless of the operation of said first carrier-operated means, and operating in response to the reception and departure from exact resonance of said carrier; and means for rendering effective said second carrier-operated means substantially concurrently with said automatic resonance adjust-l resonance with a carrier broadly selected by said,

tuning means; means for making said automatic resonance'adjusting means ineiective during the absence of a carrier;A a first carrier-responsive means, for restoring the eiectiveness of said automatic resonance adjusting means responsive to the broad selection of a carrier by said tuning means; a second carrier-responsive means, for causing said automatic resonance adjusting means to resume the ineffective condition regardless of the operation of said first carrier-responsive means, when the amount of automatic adjustment exceeds a desired value; said automatic resonance adjusting means including a discriminator unit provided with selective means providing progressively varying carrier response with departure from exact resonance; and said second carrier-responsive means including a connection to said selective means for causing resuming of the ineiiecti've condition through energy of carrior frequency derived through said selective means.

5. In a radio receiver, the combination of tuning means, automatic resonance adjusting means for bringing the receiver into substantially` exact resonance with a carrier broadly selected by said tuning means; means for making said automaticy resonance adjusting means ineffectivevduring the absence of a. carrier; a carrier-operated means for restoring the eiectiveness of said automatic adjusting means in response vto the broad selection of a carrier; a second carrier-operated means' for controlling the action of saidautomatic adjusting means, and applied tocause the resuming of the ineffective condition when the amount of automatic adjustment exceeds a, desired value; said automatic resonance adjusting means including a discriminator unit for producing a unidirectional potential in response' to the reception of the: carrier, progressivelyvarying-in magnitude with departure from exact resonance and applied to detect andV correct inaccurate tuning at least on one side-of exact resonance; and' said second carrier-operated means being -provided with an operative coupling to said discriminator unit for utilizing said-uni-directionalpotential to cause resuming of the ineiective condition as aforesaid.

6. In a radio receiver, the combination of tuning means, automatic resonance adjusting-meansA for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuningY means; meansfor making said automatic adjusting means ineffective during-thevabsence of a carrier; a carrier-operated means, for restoring the effectiveness of^saidautomatic adjusting means, in response to the broad' selectionof acarrier; Va second carrier-operated means'for controlling the action of said automatic adjusting means,4 and applied to cause the resuming of the ineffective condition when the amount of automatic adjustment exceeds a desired value; said automatic adjusting means including a discrimi nator unit providedwith selective mea-ns providing two output` points, the carrier response across one point and other increasing progressively with increasing departure from exact resonance onfone side and other respectively from 4the resonance point; uni-directional potential providing means for said second carrier-operated means, respectively coupled to said two output points, for producing potentials in response to the reception of said carrier, increasing progressively in` magnitude with departure from exact resonance said second carrier-operated means being provided with means for utilizing both of said two potentials, to cause resuming lof the ineffective condi-l tion when the magnitude of'either potential exceedsa desiredvalue.

7. A combination according to claim 6, Acharacterizedby the provision of means for additorily combining the-eiects of said two uni-directional potentials to produce afresultant potential progressivelyincreasingin magnitude in the desiredsense-withv departure from resonance `on either side of the resonance point; said second carrieroperated means being' operated by said resultant potential. f

v8; Ina radio receiver, the combination of tun' ing means; automatic resonance adjusting means for bringing the receiver into substantial-ly exact resonance with a'carrierbroadly vselected by said tuning means;` meansv for Vmaking said automatic resonance adjusting meansineffective during theV absence Aof a carrier; carrier-operated meansfor restoring the eectiveness of said automatic resonance` adjustingrmeans in response to the broad selection voi a carrier; further-carrier-operated meansfor controlling th'e'action of said auto-` matic resonancek adjusting -means,\ applied' 'to causethel resuming of the-ineffectivecondition when the amount of automatic adjustment ex-` ceeds a desiredI value;l said automatic resonanceadjusting means including a discriminator unit providedl with means for producing a uni-direce tional potentiall progressively varying in one sense and other with respect tof a reference value with departure from exact resonance on one sidefand other of the resonance point, for detecting andv correcting inaccurate tuning'on both sides -of exact resonance; -whereirrsaidfur'ther carrier'- operated means are provided with-two distinct connections to 'said uni-directional potential pro`V potential changes: derived through either connec-` tion exceeds. the associated" value of threshold.- 9. In a radio receiver, the combination of tuning means, automatic resonance adjusting means, for bringing the receiverv into substantially exactA resonance Iwith a carrier broadly selected by said tuning means; means for making said automatic resonance adjusting means ineffective during the absence ofv a carrier; a first carrieroperated i means, for restoring" the' effectiveness 'of said aua.

on one sideand other; wherein changes of one polarity ofY said potential are transferred to a first point Without phase reversal, and changes of reverse polarity are transferred to a second point through means providing reversal of:1: o'v larity; a potentiometer connected across said two points and including an element having predominant conductivity in one direction; one arm-of,

said potentiometer being predominantly permeable to potential changes of one polarity and the second arm being substantially permeable to potential changes of the reverse polarity; and said second `carrier-operated means being operatively connected to the tap of said potentiometer Whereby a potential of predetermined polarity may be derived through said connection irrespective of the polarity of said uni-directional potential of discriminator unit, to cause the resuming of the ineffective condition when the magnitude of said potential exceeds a desired threshold value.

10. In a radio receiver, the combination of tuning means, automatic resonance adjusting means for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means; means for making said automatic resonance adjusting means ineffective during the absence of a` carrier; a first carrier-operated means, for restoring the effectiveness of said automatic resonance adjusting means in response to the broad selection of a carrier; a second carrier-operated means, for causing the said automatic resonance adjusting means to resume the inelective condition when the amount of automatic adjustment exceeds a desired value; said automatic resonance adjusting means including -a discriminator unit provided with means for producing a uni-directional potential varying progressively in one sense and other with respect to a reference value with departure on one side and other from exact resonance; wherein there are provided means for impressing said potential across the input circuit of a direct-current amplier tube having a load resistor in its output circuit; means for deriving two `control potentials having relatively reversed polarity, from two points in the input and output circuits respectively of said amplifier tube, the magnitude of said two control potentials being approximately the sameV for a given amount of automatic resonance adjustment, and the potentials of said two points in the condition of exact resonance being approximately the same; a direct-current potentiometer connected across said two points and including in at least one arm an element having predominant conductivity in one direction, each arm of potentiometer being permeable to potential changes in a sense providing desired control effect; and an operative coupling including means providing a definite operating threshold, between the tap of said potentiometer and the said second carrieroperated means.

11. A combination according to claim 10, characterized in that both the anode and cathode of said amplier tube are provided with load resistors; said tube having its control grid connected to the output of said discriminator unit; and in that the said two points for deriving control potentials are the cathode and a point tapped oi from the anode circuit, respectively, further characterized in that one arm of said potentiometer comprises a diode having one electrode connected to the cathode of said tube, said tube and said diode being constructed as a unit.

12. In a radio receiving system, the combination of tuning means, an automatic resonance adjusting system for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means; means for maintaining said automatic resonance adjusting system ineffective during the absence of a carrier; a rst carrier-responsive device for restoring the effectiveness of said automatic adjusting system in response to the broad selection of a carrier; a second carrier-responsive device, for causing said automatic adjusting vsystem to-resumefthe ineffective condition, operated in accordance -wlth departure from exact resonance, and operatively associated with means for maintaining saidsecond` carrier-responsive means ineffective during the absence of a carrier and with further means for restoring the eiectiveness substantially concurrently with the restoration of the effectiveness 'of said automatic adjusting system, wherein said rst carrier-responsive deviceV comprises a channel, coupled to a desired point in `the carrieramplifying section of thereceiving system, substantially distinct from said automatic adjusting system and connected in parallel with a receiving channel serving for reproduction; said channel including selective means providing substantially symmetrical relative `response on both sides of exact resonance.

13. A combination according to claim 12, additionally characterized in that said iirst carrierresponsive device is provided with two uni-directional potential providing means fed through selective paths having unequal selectivity, said two potentials having broadly the same magnitude at exact resonance of a received carrier; and means for combining said twok potentials whereby the output effect of said device is substantially predetermined irrespective of received carrier strength and responsive in substantially predetermined fashion to departure from exact resonance.

14. In a radio receiver, the combination of tuning means; automatic resonance adjusting means for bringing the receiver into substantiallyexact resonance with a carrier broadly selected by said tuning means, and including a carrier amplifier stage; means for minimizing the gain in said stage during the absence of'a carrier; a rstcarrier-responsive means, for making ineffective the action of said gain-minimizing means, in response to the broad selection of a carrier; a secondcarrier-responsive means, for minimizing the gain of said stage regardless of the operation of said rst carrier-responsive means when the amount of mistuning exceeds a desired value; means for maintaining said second carrier-responsive means ineffective during the absence of a carrier; and means for rendering said second carrier-responsive means effective substantially concurrently with said automatic adjusting means; including a connection to said stage of the automatic adjustment means for deriving a directcurrent potential from said stage substantially reaching the value required to make said second carrier-responsive means effective after the actionfof said gain-minimizing means has been made ineffective by said rst'carrier-responsive means.

l5. In a radio receiver, the combination of tuning means; an automatic resonance adjusting system for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means; means for maintaining said automatic resonance adjusting means inelective during the absence of a carrier; a rst carrier-responsive means, for restoring Lhe effectiveness of saidk automatic adjusting system in response to the broad selection of a carrier;

said carrier-responsive means being provided automatic adjusting system, for Acausing VAsaid automatic adjusting system to resume the -ineffective condition when the amount or ymlstuning exceeds a desired low value; regardless'or the further operation of said nrst carrlereresponslve means; said second carrier-responsive means being operativelyv associated With a .threshold of comparatively hign value whereby they incident carrier energy drops comparatively easily and in response to comparatively small departure Yfrom exact resonance; below the value required toleave said. automatic adjusting system ei'ective'; and means ior maxing said second carrier-*responsive means inenective during the absence of acarrier and for rendering said second means effective substantially concurrently with said automatic adjustmg system. l

` 16. In a radio receiver, the combination of tunmg means; an automatic resonance adjusting system for bringing the receiverinto substantially exact resonance witha carrier broadly selected by said tuning means, and including'adiscriminator unit provided with selective means for detecting departure i'rom exact resonance; means for making ineffective an element said automatic resonance adjusting system, inthe absence of a carrier, to disable normally said systern; a rst carrier-responsive means, for deriving a uni-directional potential in'response to the broad selection of a carrier utilized to restore the eectiveness of the aforesaidA element; a second carrier-responsive means, including Va connection to said discriminator unit, for deriving a second uni-directional potential in accordance with the amount of automatic resonanceadjustment, utilized to cause the resuming of the disabledcondition of said element when said amount exceeds a desired value; wherein there are provided means for combining the control eifectsupon said element of said two uni-directional potentials whereby increased eifectiveness of carrier re,- ception of said first carrier-responsive means tends to neutralize the action of said means disabling the said element; and said second potential tends to enhance the action of said means disabling the aforesaid element.

17. In a radio receiver, the combination of tuning means; an automatic resonance Vadjusting system for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means and including a discriminator unit and a carrier-ampliiier stage; means for minimizing the gain of said stage in the absence of a carrier; a rst carrier-responsive means for deriving a unidirectional potential in response to the broad selection. of a carrier, utilized to restore the gain of said stage; a second carrier-responsive potential having a substantially predetermined amplitude'after said automatic adjusting system is rendered eifective;a second carrier-responsive means including aconnection to said discriminator unit and output of said stage, for deriving a second uni-directional potential in accordance with theamount of automatic resonance adjustment, utilized to cause the resuming of the condition of minimized gain when said amount exceeds a desired value; wherein there are provided means for algebraically superposing said two potentials; and an operative coupling with the aforesaid gain-minimizing means for utilizing the algebraic sum of said two potentials, said icoupling including means providing a denite threshold of operation.

' -V18. Ina radioreceiver, the combination of tuning means`; an automatic resonance adjusting system for bringing the receiverinto substantiallyexact resonance with a carrier broadly selected by said tuning means; means for rendering ineiective anlel'e'ment in said automatic adjusting system inthe absence of a carrieigto disable-normally said' system; a rst carrier-ree sponsiv 'meansf'for deriving a unidirectional potential in response to the broad selection of a carrier, utilized through means providing a threshold? of operation"v toV restore' the eifective- 'ness' of the aforesaid element when, and only when, saidpotentialexceeds a desired compara- `tively low threshold; a second carrier-responsive means, -made effective Vsubstantially vconcurrently with saidautomatic adjusting system, for deriving a second uni-directional potential, in accordance Vwith departure from'y exact resonance, utilized to cause the resuming of the disabled condition'when said'amount of departure exceeds a desired value; lwherein there is provided an operative coupling between said rst carrierresponsive means and theroutput of said second carrier-"responsive means for eifectively altering thel relation of the amplitude of said flrst carrier-responsive potential and associated operative threshold to raise said threshold to a comparatively more unstable value.

19.y In a radio receiver, the combination of tuning means; an automatic resonance adjusting system for bringing thel receiver into substantially exact resonance with a carrier broadly se'- lected by said tuning means, and including a `discriminator unit; means for rendering inef fective an element in said automatic adjusting system inthe absence of a carrier, to disable normally said system; a rst carrier-responsivev means, for deriving a unidirectional potential in response to theV broad selection of a carrier, utilized through means providing a threshold of operation, to restore the effectiveness of the aforesaid element only when said potential exceeds a desired comparatively low threshold value; a second carrier-responsiveV means, .in` cluding a connection to said discriminator unit, for deriving a second uni-directional potential in accordance with the amount of automatic ad` justment, utilized to cause the resuming of the disabled condition when said amount exceeds a desired value; wherein there is provided an operative coupling between said first carrier-responsive means and the output of said second carrier-responsive means for effectively altering the relation of the amplitude o-f said first potential and associated operative threshold to progressively urge said threshold toward an unstable condition. Y 20. In a radio receiver, the combination of tuning means; an automatic resonance adjusting system for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means and including a discriminator unit and a carrier-amplier stage; means for minimizing the gain of said stage' in the absence of a carrier; a first carrier-responsive device for deriving a uni-directional potential'` in response to the broad selection of a carrier, for restoring the gain of said stage only when'said potential exceedsa desiredthreshold Value; said 'device including a selective circuit operatively associated with means for raltering the reso-nant properties of said circuit whereby effectively .to alter the adjustment offsaid rst `potential rela'-,Y tive to said operative threshold value; a second carrier-responsive means, provided with a coupling to said discriminator kunit,forderiving a uni-directional potential Aapplied to cause-said stage toresume the condition of minimized gain when-the amount of automatic adjustment exceeds a desired value; wherein there is provided an operative coupling between `said second carrier-responsivepotential and said resonance a1- tering means, to make said threshold progressively unstable.

2l.Y A combination as defined in claim 20, wherein said discriminator unit is adapted to produce a uni-directional potential progressively Varying in one sense and `another with respect to a reference value with departure on one side and the other from exact resonance, applied to vdetect and `correct inaccurateV tuning on both sides `of exact resonance; and said resonance altering means is adapted to produce' mistuning 'of said selective circuit on one side and other vfrom a reference frequency substantially corresponding a exact resonance; and wherein said fresonance altering means is provided with an operative coupling tosaid discriminator unit and second carrier-responsive means, for utilizing the said uni-directional potential to cause progressive mistuning.

22. In a radio receiver, the combination of tuning means; automaticvresonance adjusting means for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means, and including a carrier amplifier channel and a discriminator unit; means for minimizing the gain in said channel during the absence of a carrier; a first carrier-responsive means, for rendering ineffective the action of said gain-minimizing means in response to the broad selection of a carrier; a second carrier-responsive means, including means for producing a second potential, responsive to the amount of automatic adjustment, derived through said channel and discriminator unit and utilized to cause said channel to resume a condition of minimized gain'when the amount of departure from exact resonance in the supposed absence of said automatic adjusting means exceeds a desired value; characterized in that the control connection for utilizing said second potential is provided with auxiliary means providing a comparatively sharp rise of the control eiect when said potential exceeds a threshold value, whereby to cause said resuming of the condition of minimized gain comparatively abruptly, and wherein furthermore said control connection includes a time-delaying network for maintaining said potential effective for a brief while after said channel is caused to resume said condition.

23. In a radio receiver, the combination of tuning means; automatic resonance adjusting means for bringing the receiver intosubstantially exact resonance with a carrier broadly selected by said tuning means and including a carrier -amplifier stage and a discriminator unit; means for minimizing the gain in said stage during the absence of a carrier; a first carrier-responsive means for producing a potential in response to the broad selection of a carrier; means for utilizing said potential to control the gain of said stage in a sense such as to make ineffective said gain minimizing means; a further carrier-responsive means for producing a second potential, responsive to the amount of automatic adjustment, derived through said stage and discriminator unit and utilized to control the gain of said stage in a sense such as to sustain the action of; said gain 75| ,minimizing means; wherein there are provided means for combining the control effects upon ithe aforesaid utilizing meansin such a way as to produce aresultant control potential; and auxiliary means between. Said last-named potential.- Cmbilg. means arid `the aforesaid Vutilizing means for providing a r`comparatively sharp Qheneef the, vcontrol eiet when said resultant potential exceeds a threshold value whereby controlin said utilizing means may becomparatively abruptly," and wherein there is furthermore provided a` time-delaying net-work between said potential-combining"means" and` said utilizing means; for maintaining the effect of said resultant'potential for a brief While after said'stage is caused to resume /thecondition of minimized gain. y

24. In a radio receiver, the combination of tuning means; an automatic adjusting system for bringing the receiver into substantially exact resonance with a carrier broadly selected by said tuning means; Ymeans for normallymaintaining saidautomatic adjusting Vsystem ineffective; a carrier-responsive means effective at all times for producing a potential responsive to mistuning, said means including a 'selective circuit making said means responsive to comparatively small departure from exact resonance; means having a denite threshold of operation for utilizing said potential tomake said automatic adjusting system effective, said threshold being sufficiently low tobe exceeded in response to the broad selection-of a carrier wave, whereupon said automatic adjusting system may bring the receiver frequency close to exact resonance; means for deriving an auxiliary potential of substantially predetermined amplitude in response to the condition of effectiveness of said automatic adjusting system; means for utilizing said auxiliaryV potential to counteract the influence of said rstnamed potential responsive to Ymi'stuning upon saidV means having a threshold of operation whereby to leave a comparatively small excess influence of said first-named potential despite the improved resonance condition as set forth so that said means having a threshold of operation may become responsive to rsmall departure from exact resonance; and a time-delaying filter between said means for deriving said auxiliary potential and the means for utilizing said auxiliary potential.

25. In a radio receiver, the combination of tuning means; automatic adjusting means for bringing the receiver into substantially exact resonance with a carrier broadlyselected ,by said timing means including a discriminator for deriving a potential; means for normally maintaining said automatic adjusting system ineffective; a first carrier-responsive means, for restoring the effectiveness of said automatic adjusting means in response to the broad selection of a carrier; a second carrier-responsive means, for causing'said automatic adjusting system to resumefthe ineective condition when the amount of mistuning exceeds a desired comparatively very low value; said secondy means being normally maintained ineiective and being rendered effective substantially concurrently with said automatic adjusting means; characterized by the provision of means at the output of said discriminator unit for substantially limiting the eiective magnitude of said potential of the discriminator, and further characterized in that said means includes a diode associated with a biasing threshold.

' EGON NICOLAS MULLER. 

